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TitleSpatio-temporal challenges in dating orogen-scale shear zones: the case of the Himalayan Main Central Thrust
AuthorBraden, Z; Godin, L; Kellett, D AORCID logo; Yakymchuk, C
SourceTectonophysics vol. 774, 228246, 2019 p. 1-23,
Alt SeriesNatural Resources Canada, Contribution Series 20190268
Mediapaper; on-line; digital
File formatpdf; html; xlsx (Microsoft® Excel®)
AreaHimalayas; Simikot; Chuwa River; Humla Karnali River; Tamcha; Dailekh; Chaudhabis River; Nepal
Lat/Long WENS 81.0000 84.0000 30.5000 28.0000
Subjectsgeochronology; tectonics; igneous and metamorphic petrology; mineralogy; geochemistry; Science and Technology; Nature and Environment; shear zones; tectonic evolution; orogenies; metamorphism; deformation; shearing; faulting; displacement; pressure-temperature conditions; crystallography; radiometric dating; argon argon dating; mica; zircon dates; bedrock geology; structural features; faults, thrust; faults, normal; faults, strike-slip; folds; anticlines; klippen; lithology; sedimentary rocks; igneous rocks; intrusive rocks; granites; foliation; lineations; thermobarometry; spectroscopic analyses; modelling; Himalayan Main Central Thrust; Greater Himalayan Sequence; Lesser Himalayan Sequence; Main Himalayan Thrust; Main Frontal Thrust; Main Boundary Thrust; Gurla-Mandhata Humla Fault; South Tibetan Detachment; India Craton; Tethyan Sedimentary Sequence; Karnali Klippe; Phanerozoic; Cenozoic; Tertiary
Illustrationslocation maps; geoscientific sketch maps; cross-sections; columnar sections; photomicrographs; stereonets; graphs; diagrams; phase diagrams; tables; bar graphs; schematic cross-sections
ProgramScience laboratory network
Released2019 11 09
AbstractThe spatial extent and dynamic evolution of orogen-scale shear zones renders the determination of the timing and duration of ductile deformation and displacement challenging. The Himalayan Main Central thrust (MCT) is one of the most widely studied thrust-sense shear zones in the world, particularly regarding its timing, and yet 'when was the MCT active' remains an outstanding question. New and existing pressure-temperature-deformation-time (P-T -d-t) data from three well-exposed structural sections of the MCT in western Nepal, which together represent>100 km in the direction of tectonic transport, are compared to examine how spatio-temporal variations in recorded shear complicate the answer to this question. Metamorphic P-T data, quartz microstructures and quartz crystallographic preferred orientation suggest that all three MCT transects underwent similar shear conditions and kinematic behaviour. However, each shear-zone transect reached peak T conditions during different windows of time. Furthermore, 40Ar/39Ar thermochronology on white mica reveals contrasting timing for the end of ductile deformation. When combined with published monazite and zircon dates, our data indicate that the record of ductile deformation on the MCT is diachronous in the transport direction and a single shear zone transect is thus not representative of the timing and duration of deformation on the MCT even for a narrow along-strike position. This study represents the first attempt to track ductile deformation in the direction of thrust propagation at this spatial extent on an orogen-scale thrust system. These results clearly point to the need to study the temporal evolution of shear zones over significant distances in the direction of tectonic transport, acknowledge potential gaps in the petrochronological record, and incorporate P-T -d data when using shear zone timing and duration results to make regional conclusions.
Summary(Plain Language Summary, not published)
Determining the absolute timing of ancient shear zones remains a scientific challenge because the samples we collect and the minerals within those samples that we can date using radiometric dating methods preserve an incomplete record of the event we are trying to date. However, small, spatially restricted sample sets are routinely used to date shear zones. In this study, we examine a spatially extensive geochronological dataset from a well-studied shear zone in the Himalayan orogen, the Main Central thrust. We show the different partial histories of shear recorded in different parts of the shear zone to demonstrate the need for more spatially extensive studies, and the need for more discussion about potential data gaps in existing studies.

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